TECHNOLOGY SUBSTRATES


variety of thin films of 2-inch semi-polar material, based on orientations such as {1011}, {1122}, and {2021}.


Although producing these thin films is important, for practical applications bulk substrates are needed, and that means the deposition of thick, crack-free layers of GaN that are free from anomalous growth. We have done just that by turning to new sample structures for the fabrication of a semi-polar {2021} GaN substrate using SiO2


stripe-masked templates (see Figure 4). Optimised SiO2 striped


masks were prepared in the direction perpendicular to the a-axis on a 2-inch {2021} GaN templates, prior to the of growth of a 1.4 mm-thick GaN layer at a deposition rate of 350 mm/h. The growth tool employed is a vertical-flow-type HVPE apparatus equipped with liquid gallium source, hydrogen chloride, ammonia, nitrogen and a hydrogen gas cylinder.


The SiO2


stripes play a crucial role in this substrate formation process. Without the striped mask, GaN forms a rough and cracked surface. But when it’s there, the surface is far smoother, with roughening originating from unintentional anomalous growth regions on the template – note that these regions were completely embedded during selective area growth by HVPE. What’s more, the SiO2


mask is


effective on other planes, such as {1011} or {1122}, when growing GaN on patterned sapphire.


Another attractive feature of our approach is the effective self-separation of the patterned sapphire and the GaN film. In comparison, typical methods employed for separating a GaN layer grown on a foreign substrate are more involved, such as mechanical polishing, laser or chemical lift-off, or self-separation via the growth of an intentional interlayer. With these more common methods, an additional process is required before or after HVPE growth of GaN.


When a layer of GaN is grown on sapphire, as the wafer cools thermal stress is induced in both materials, due to a difference in the thermal expansion coefficients, and this leads to a maximum shear stress at the heterointerface. We take advantage of that with our approach:


Figure 5. As grown surface of various orientations of GaN. Normarski microscope images of a {2021} GaN layer are also shown. The red arrows show the cracks


Figure 4. Experimental set-up. (a) A 1.4 mm-thick GaN layer was grown on a SiO2


masked template.


(b) HVPE apparatus can accommodate a 6-inch substrate. Four, 2-inch wafers were loaded in this work


Figure 6. A GaN substrate formed by chemical mechanical polishing. (a) Photograph of 2 inch c-plane, {1122} and {2021} GaN wafer. (b) Surface morphology of the chemical-mechanical polished {2021} GaN substrate measured by a scanning white light interferometer


50 www.compoundsemiconductor.net March 2014


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